The present invention pertains to a device for measuring the flow of a fluid flowing in a fluid channel.
Thermal flow measurement methods have been known in which thermocouples are used to determine the transport of heat.
A device for measuring the flow by means of a thermocouple has become known from DE 195 01 347 C2. The junction points of the thermocouple are arranged one behind another in a fluid channel and are heated to an increased working temperature compared with the temperature of the fluid by means of an a.c. power source. The voltage of the thermocouple, which is obtained during the flow through the fluid channel from the difference in the cooling of the junction points, is used as the measured variable. The a.c. voltage components, which are superimposed to the thermocouple voltage due to the a.c. heating of the thermocouple, are eliminated by a circuit.
The drawback of the prior-art device is that the junction points are located directly in the fluid channel and are subject to different stresses. Thus, there is a risk that the junction points are damaged by foreign bodies in the fluid, or increased corrosive effects occur in the case of corrosive media, because the junction points consist of different materials for design reasons.
A thermal flowmeter with a thermocouple, in which the fluid channel consists of a heated, metallic tube section, has been known from DE 31 12 144 A1. The junction points of the thermocouple are formed at the ends of the tube sections by different material pairs between the lead wires and the tube material. Both the tube section and the junction points are heated to an increased working temperature compared with the temperature of the fluid by means of a heating section located on the tube section in the form of a plurality of film resistors. The drawback of the prior-art device is that a separate heating element is needed for heating up the tube section.
The basic object of the present invention is to improve a device of the above-mentioned type such that the heating of the fluid channel is possible without additional components and the thermocouple is arranged in a protected manner.
According to the invention, a device for measuring the flow of a fluid located in a fluid channel is provided. The device includes a metallic tube of a predetermined thermoelectric potential as a fluid channel. An a.c. power source is connected to a partial section of a tube in such a manner that the partial section is heated as a resistor element to an increased working temperature compared with the temperature of the fluid. At least one thermocouple is provided with junctions formed in the area of the partial section in such a manner that at least two wires with another thermoelectric potential are connected to the tube. A measuring circuit evaluates the thermocouple voltage of the thermocouple. The measuring circuit includes a circuit which eliminates the a.c. voltage component superimposed to the thermocouple voltage as a consequence of the a.c heating. The thermocouple voltage yields the measured variable corresponding to the flow.
The advantage of the present invention is essentially that, on the one hand, the thermoelectric potential of the lead wires is adjusted to the thermoelectric potential of the tube material due to the use of a metallic tube in combination with lead wires such that the junction points of the thermocouple are formed directly at the contact points between the lead wires and the tube material and that, on the other hand, the tube can be brought to the working temperature together with the junction points due to the fact that the tube is directly integrated in an electric circuit, which is connected to an a.c. power source. The partial section of the tube that is connected to the a.c. power source now acts as a resistor element, which is heated by the a.c. power source. Another advantage is that the tube section heated in this manner can be adapted to different requirements in an especially simple manner by using tube sections of different length for the heating depending on the particular application. As a result, different measurement ranges can be obtained for the flow measurement in an especially simple manner. By contrast, the tube sections according to the state of the art must be provided individually with correspondingly adapted film resistors in the case of prior-art tube sections of different length in order to achieve a uniform heating. Temperature gradients between the film resistors or between the film resistors and the junction points cannot always be avoided because only a punctiform heating of the tube section is possible by means of individual film resistors.
The device according to the present invention is not limited to the flow measurement of a particular fluid. The device according to the invention is also suitable generally for the flow measurement of gases and liquids.
It is advantageous to arrange the junction points within the heated partial section of the tube. If, e.g., a larger partial section of the tube is heated, favorable distances can be experimentally determined for the junction points within the partial section. The junction points are preferably arranged in the area of the ends of the partial section. The junction points can thus also be located outside the partial section.
It is particularly advantageous to use the wires that lead to the junction points as the power lead for the a.c. power source. As a result, only two contacts must be established to the tube.
The suitable material pairs between the tube and the wires are selected from the group of the materials chromel/constantan, copper/constantan and iron/constantan.
An exemplary embodiment of the present invention is shown in the figure and will be explained in greater detail below. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.